Antennas started to be developed at the end of the nineteenth century based on the fundamental laws of electromagnetism postulated by James Clerk Maxwell in 1864. The invention of the first antenna has to be attributed to Heinrich Hertz in 1886 who demonstrated the transmission through air of electromagnetic waves. In the mid-1940's the fundamental restrictions regarding the reduction in size of antennas were shown with respect to wavelength and at the beginning of the sixties appeared the first frequency-independent antennas (E. C. Jordan, G. A. Deschamps, J. D. Dyson, P. E. Mayes, “Developments in Broadband Antennas,”IEEE Spectrum, vol.1, pp. 58-71, Apr. 1964; V. H. Rumsey, Frequency-Independent Antennas. New York Academic, 1966; R. L. Carrel, “Analysis and design of the log-periodic dipole array,” Tech. Rep. 52, Univ. of Illinois Antenna Lab., Contract AF33 (616)-6079, October 1961; P. E. Mayes, “Frequency Independent Antennas and Broad-Band Derivatives Thereof”, Proc. IEEE, vol.80, no.1, January 1992). At that time proposals were made for helical, spiral, log-periodic arrays, cones and structures defined exclusively by angle pieces for the implementation of broadband antennas.
Antenna array theory goes back to the works of Shelkunoff (S. A. Schellkunhoff, “A Mathematical Theory of Linear Arrays,” Bell System Technical Journal, 22,80), among other classic treatises on antenna theory. Said theory establishes the basic design rules for shaping the radiation properties of the array (principally its radiation pattern), though its application is restricted mainly to the case of mono-band arrays. The cause of said restriction lies in the frequency behaviour of the array being highly dependent on the ratio between the distance between elements (antennas) of the array and the working wavelength. Said spacing between elements is usually constant and preferably less than one wavelength in order to prevent the appearance of diffraction lobes. This implies that once the spacing between elements is fixed, the operating frequency (and the corresponding wavelength) is also fixed, it being particularly difficult that the same array work simultaneously at another higher frequency, given that in that case the magnitude of the wavelength is less than the spacing between elements.
The log-periodic arrays suppose one of the first examples of antenna arrays capable of covering a broad range of frequencies (V. H. Rumsey, Frequency-Independent Antennas. New York Academic, 1966; R. L. Carrel, “Analysis and design of the log-periodic dipole array,” Tech. Rep. 52, Univ. Illinois Antenna Lab., Contract AF33 (616)-6079, October 1961; P. E. Mayes, “Frequency Independent Antennas and Broad-Band Derivatives Thereof”, Proc. IEEE, vol.80, no.1, January 1992). Said arrays are based on distributing the elements that constitute it in such a manner that the spacing between adjacent elements and their length vary according to a geometric progression. Although said antennas are capable of maintaining a same radiation and impedance pattern over a broad range of frequencies, their application in practice is restricted to some concrete cases due to their limitations regarding gain and size. Thus for example, said antennas are not employed in cellular telephony base stations because they do not have sufficient gain (their gain is around 10 dBi when the usual requirement is for about 17 dBi for such application), they usually have linear polarisation whilst in said environment antennas are required with polarisation diversity, their pattern in the horizontal plane does not have the width necessary and their mechanical structure is too bulky.
The technology of individual multiband antennas is markedly more developed. A multiband antenna is understood to be an antenna formed by a set of elements coupled to each other electromagnetically which interact with each other in order to establish the radio-electric behaviour of the antenna, behaviour which with respect to radiation and impedance patterns is similar in multiple frequency bands (hence the name multiband antenna). Numerous examples of multiband antennas are described in the literature. In 1995 antennas of the fractal or multifractal type were introduced (the coining of the terms fractal and multifractal is attributable to B. B. Mandelbrot in his book The Fractal Geometry of Nature, W. H. Freeman and Co. 1983), antennas which by their geometry have a multifrequency behaviour and, in determined cases, a reduced size (C. Puente, R. Pous, J. Romeu, X. Garcia “Antenas Fractales o Mulitfractales”, (Spanish patent P9501019). Subsequently multi-triangular antennas were introduced (Spanish patent P9800954) which could work simultaneously in the GSM 900 and GSM 1800 bands and, more recently, multilevel antennas (Patent PCT/ES99/00296), which offer a clear example of how it is possible to shape the geometry of the antenna in order to achieve a multiband behaviour.
The present invention describes how multiband antennas can be combined in order to obtain an array that works simultaneously in several frequency bands.
A Multiband Interleaved Array (MIA) consists of an array of antennas which has the particularity of being capable of working simultaneously in various frequency bands. This is achieved by means of using multiband antennas in strategic positions of the array. The disposition of the elements that constitute the MIA is obtained from the juxtaposition of conventional mono-band arrays, employing as many mono-band arrays as frequency bands that it is wished to incorporate in the Multiband Interleaved Array. In those positions in which one or various elements originating in the conventional mono-band arrays coincide, a single multiband antenna (element) shall be employed which covers simultaneously the different bands. In the remaining non-concurrent positions, it can be chosen to employ also the same multiband antenna or else recur to a conventional mono-band antenna which works at the pertinent frequency. The excitation at one or various frequencies of each element of the array depends therefore on the position of the element in the array and is controlled by means of the signal distribution network.